The disclosure relates to scratch-resistant articles with retained optical properties and more particularly to articles that exhibit low color shift when viewed at different incident illumination angles and a high hardness.
Cover substrates are often used in consumer electronic products to protect critical devices within the product, to provide a user interface for input and/or display, and/or many other functions. Such consumer electronic products include mobile devices, such as smart phones, mp3 players and computer tablets. These applications and others also often demand durable (e.g., scratch-resistant) cover substrate, which also has strong optical performance characteristics. Often, the cover substrate includes glass for this purpose.
Strong optical performance in terms of maximum light transmittance and minimum reflectance are required in cover substrate applications. Furthermore, cover substrate applications require that the color exhibited or perceived, in reflection and/or transmission, does not change appreciably as the viewing angle (or incident illumination angle) is changed. This is because, if the color in reflectivity or transmission changes with viewing angle to an appreciable degree, the user of the product incorporating the cover glass will perceive a change in the color or brightness of the display, which can diminish the perceived quality of the display. Of these changes, a change in color is often the most noticeable and may be the most objectionable to users.
Known cover substrates include glass and film combinations that often exhibit scratches after use in harsh operating conditions. Evidence suggests that the damage caused by sharp contact that occurs in a single event is a primary source of visible scratches in such glass-film cover substrates used in mobile devices. Once a significant scratch appears on the cover substrate, the appearance of the product is degraded since the scratch causes an increase in light scattering, which may cause significant reduction in brightness, clarity and contrast of images on the display. Significant scratches can also affect the accuracy and reliability of touch sensitive displays. These scratches, and even less significant scratches, are unsightly and can affect product performance.
Single event scratch damage can be contrasted with abrasion damage. Cover substrates do not typically experience abrasion damage because abrasion damage is typically caused by reciprocating sliding contact from hard counter face objects (e.g., sand, gravel and sandpaper). Instead, cover substrates used in display applications typically endure only reciprocating sliding contact from soft objects, such as fingers. In addition, abrasion damage can generate heat, which can degrade chemical bonds in the film materials and cause flaking and other types of damage to the cover glass. In addition, since abrasion damage is often experienced over a longer term than the single events that cause scratches, the film material experiencing abrasion damage can also oxidize, which further degrades the durability of the film and thus the glass-film laminate. The single events that cause scratches generally do not involve the same conditions as the events that cause abrasion damage and therefore, the solutions often utilized to prevent abrasion damage may not be applicable to prevent scratches in cover substrates. Moreover, known scratch and abrasion damage solutions often compromise the optical properties.
Accordingly, there is a need for new cover substrates, and methods for their manufacture, which are scratch resistant and have good optical performance.